Unix return code example

Shell Programming and Scripting
1

Hi,

Does anyone here can guide me to understand how is return code works in a parent-child relation with a simple example?
I have a request to build the script with return code in a child script, but i want to understand how does child script can return a code to the parent, stated if its succeeded or failed (know that 0 is success, anything more than 0 is failed).
Thanks for any advise given.
Appreciate help in advance.
best regards

2

A script sets the return value using the exit command. This small script exits with a success (0) if the file named on the command line is found:

#!/usr/bin/env ksh
if [[ -f ${1:-no-such-file} ]]
then
   exit 0
else
   exit 1
fi

If no file is given on the command line, or the file isn't found it returns 'bad.'

3

Since the OP was asking about exit status's in regards to child processes... I thought I would add this.

Consider two shell scripts, parent and child:

PARENT (test-parent.sh):

#!/bin/sh


# bash and ksh will expand '$?' to be the exit status of the previous command
# we can use that to get a child scripts exit status
# If you have many child processes and they are running
# in parallel you will want to look into the trap statement.
# trap will allow you to get info back from the child using something like
# trap "command to run on trap" CHLD
# where CHLD is the exit status of the child process

echo "I am the parent script."
echo
echo "I am now calling the child script..."
echo
echo "###############################################"
./test-child.sh
XSTAT=$?
echo "###############################################"
echo
echo "We have now obtained the exit status of the child (exit status: ${XSTAT})."
echo "We can determine what to do (exit with bad exit code or live on)."
echo
if [ "${XSTAT}" -gt 0 ]; then
        echo "Bad exit from child... exiting."
        exit ${XSTAT}
else
        echo "The child exited gracefully with exit status '${XSTAT}'"
        echo "We can now proceed doing other tasks as usual."
fi
echo "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
echo "Weeeeeee other tasks."
exit ${XSTAT}

CHILD(test-child.sh):

#!/bin/sh

echo "I am the child script."
echo -n "What would you like my exit status to be: "
read XSTAT
echo "Exiting with exit status: ${XSTAT}"; exit ${XSTAT}

Now see the output when run...

BAD EXIT STATUS (not 0)

[user@host ~]$ ./test-parent.sh 
I am the parent script.

I am now calling the child script...

###############################################
I am the child script.
What would you like my exit status to be: 3
Exiting with exit status: 3
###############################################

We have now obtained the exit status of the child (exit status: 3).
We can determine what to do (exit with bad exit code or live on).

Bad exit from child... exiting.

GOOD EXIT STATUS (0)

[user@host ~]$ ./test-parent.sh 
I am the parent script.

I am now calling the child script...

###############################################
I am the child script.
What would you like my exit status to be: 0
Exiting with exit status: 0
###############################################

We have now obtained the exit status of the child (exit status: 0).
We can determine what to do (exit with bad exit code or live on).

The child exited gracefully with exit status '0'
We can now proceed doing other tasks as usual.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Weeeeeee other tasks.
4

sorry of the dummy question:
if [ "${XSTAT}" -gt 0 ]; then

is -gt means not equal to 0?

5

Have a read the Man Page for test.